The described embodiments relate generally to the field of conversion of direct current (DC) to alternating current (AC) and vice versa, and more specifically, relate to converters (inverters and rectifiers) that are multilevel.
Such power conversion equipment is particularly useful for renewable power generation systems such as wind and solar power generation systems. Generally a wind turbine includes a rotor that includes a rotatable hub assembly having multiple blades that transform wind energy into a mechanical rotational torque that drives one or more electrical generators via the rotor. With the rapid growth of grid-connected renewable power generation systems, renewable power penetration into the power grid may have a significant impact on the grid voltage and frequency. It is desirable to regulate the voltage and frequency of the AC power at the output of the power generation system. In wind turbine embodiments, one or more power converters are coupled to the generator to convert the power to provide power with an appropriate frequency and voltage for the utility grid.
FIG. 1 shows a phase (or leg) of a conventional active neutral point clamped five-level (ANPC-5L) converter 10. The phase of the ANPC-5L converter 10 includes a DC link 12, a first cell 14, a second cell 16, a third cell 18, and a phase capacitor (also called flying capacitor) cph. The DC link 12 includes a first capacitor c1 and a second capacitor c2. The first cell 14 includes a first switching element s1, a second switching element s2, a third switching element s3, and a fourth switching element s4. The second cell 16 includes a fifth switching element s5 and a sixth switching element s6. The third cell 18 includes a seventh switching element s7 and an eighth switching element s8. The switching elements s1-s8 comprise insulated gate bipolar transistors (IGBTs) in one example.
In the ANPC-5L converter 10, the phase capacitor cph is kept charged to half the voltage of the first capacitor c1 or the second capacitor c2 in the DC link. As the voltage of each capacitor c1 and c2 is typically equal, the phase capacitor cph is thus charged to one quarter of the total DC-link voltage Vdc. The number of output phase voltage levels of the converter 10 will be determined by switching different combinations of all of the switching elements s1-s8. The following table shows the output voltage Van of the converter 10 based on different combinations of the switching elements s1-s8 wherein 1 indicates a switching element being conducting and 0 indicates a switching element being non-conducting.
s1s2 s3s4s5s6s7s8Vanvector01010101−VV001010110−V/2V101011001−V/2V2010110100V3101001010V410100110V/2V510101001V/2V610101010VV7According to above table, the output voltage Van of the converter 10 may comprise eight voltage vectors V0-V7 in five voltage levels (−V, −V/2, 0, +V/2, +V). For example, when the switching elements s2, s4, s6, s8 are turned on and the switching elements s1, s3, s5, s7 are turned off, the output voltage Van is equal to −V (see the bold path in FIG. 1). The other levels may be calculated in a similar manner.
Even though the conventional ANPC-5L converter 10 is successful in converting DC to AC (or AC to DC), the selection of the voltage of the phase capacitor cph is limited. Furthermore, the power rating of the conventional ANPC-5L converter 10 may be not optimal, which can increase costs by requiring additional high voltage devices, such as additional high voltage switching elements.
Therefore, it is desirable to provide a new multilevel power source converter topology configuration to increase the selection of the phase capacitor and reduce costs of using high voltage devices while maintaining a high efficiency and generating waveforms of high quality.